1) Field of the Invention
The present invention generally relates to a time of flight method, apparatus and device for measuring a parameter of a flow, and particularly for measuring fluid flow rates using a pseudo-random sequence of tracer elements injected into a flow channel at a first location and detected at a second location.
2) State of the Art
Parameter measurement of a flowing fluid or gas has numerous applications in residential and commercial setting. One such parameter of particular interest is flow rate. Flow rate measurements are central to a variety of industries and applications. In some applications the flow to be measured may be a high volume flow through a large diameter pipe or fluid channel, or in the alternative may be a low volume flow through a micro fluidic channel. Additionally, these flows may be gas flows, liquid flows or some combination of both gas and liquid flow. Furthermore, the flow may be a single phase or multi-phase flow. While these various flows span numerous applications, one such environment and application is the oil and natural gas industry. The oil and natural gas industry encounters a variety of the aforementioned flow types in a variety of settings, spanning downhole reservoirs flow rate analysis to production logging flow rate analysis, to monitoring the injection of synthetic fluids (such as fracturing fluids) into the rock formation, to monitoring flow within a flowline or within channels inside a tool, to surface operations. These various operating environments each present a variety of flow analysis challenges.
In a downhole flow analysis environment, for example, the naturally occurring hydrocarbon fluids may include dry natural gas, wet gas, condensate, light oil, black oil, heavy oil, and heavy viscous tar. In addition, there may be flows of water and of synthetic fluids, such as oils used in the formulation of drilling muds, fluids used in formation fracturing jobs etc. Each of these individual fluids presents vastly different physical properties, yet all may pass through a single flow channel for measurement.
As the economic value of a hydrocarbon reserve, the method of production, the efficiency of recovery, the design of production hardware systems, etc., all depend upon a number of flow parameters, such as physical properties, phase behavior and flow rates of the fluid, it is important that the flow parameters be determined accurately.
Additionally, in a production logging environment it is preferred to have knowledge of the flow velocities for different phases such as oil, water and gas at different places axially and radially in the production pipe so that one may have a proper understanding of oil production and well development. Ideally, a flow measurement should cover a wide range of flow rates, should work irrespective of fluid composition or phase (oil, gas or water), and should provide a local measurement (so that a map of the flow across the borehole can be created) without perturbing flow. A useful addition to these elements would be the potential to apply the same measurement scheme in a miniaturized geometry, such as a micro fluidic device. The assignee of this application has provided a commercially successful production logging tool, the FSI (Flow Scan Imager) which is capable of performing flow rate analysis of formation fluids. The present invention may be incorporated into this tool, or any other production logging environment, for the analysis of formation fluids.
Several measurement principles have been attempted in the past to measure such flows for the hydrocarbon industry and other industries. For gas flow, thermal anemometers are widely used. Spinners are being used in production logging to measure liquid and gas flow. Venturi pressure drop, Coriolis flowmeters, electromagnetic, cross-correlation flow meters, gamma-ray absorption, gradio-manometer densitometers, local electrical and fiber-optic sensors have all been applied to measurements of single- or two-phase flow. In addition, techniques based on thermal tracers and stochastic techniques have been developed for measuring blood flow velocity inside arteries as recited in U.S. Pat. No. 4,507,974 to Yelderman, which is herein incorporated by reference. Micro-scale time-of-flight sensors using thermal tracers, albeit without the added benefit of using correlated sequences, have been reported in the scientific literature (E. Meng, Y.-C. Tai: “A PARYLENE MEMS FLOW SENSING ARRAY”, Proceedings of the 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, June 8-12, p. 686, 2003). Inherent in many of these existing techniques, however, is the inability of unobtrusively measure a parameter of a fluid, such as flow rate, in an accurate manner regardless of the flow rate and composition of the flow.